The clinical utility of tissue engineering depends upon our ability to direct cells to form tissues with characteristic structural and mechanical properties across different hierarchical scales. Ideally, an engineered graft should be autologous and tailored to (re)establish the structure and function of a lost or damaged tissue. Engineered tissues with such a high level of structural and functional complexity would also foster fundamental research by serving as physiologically relevant models for controlled quantitative studies. The application is for tissue engineering of human stratifed grafts suitable for mandibular condyle replacement, a tissue of great clinical interest and an excellent model for studies of stem cell differentiation and functional assembly into craniofacial tissues. We hypothesize that craniofacial structures with physiological gradients of structural and mechanical properties can be grown in vitro by biophysical regulation of adult human stem cells. We thus propose to engineer mandibular tissue grafts by culturing adult human stem cells on specialized scaffolds in advanced bioreactors. Aim 1 will focus on the development of modular bioreactors with environmental control, interstitial flow, mechanical loading and imaging compatibility. Aim 2 will focus on the development of silk protein scaffolds with spatial gradients of immobilized growth factors. In Aim 3, the advanced scaffolds and bioreactors will be utilized to engineer human tissue constructs with structural and mechanical properties resembling those of native condyles. The resulting grafts are expected to have sufficiently high fidelity for use in studies of stem cell responses to genetic and environmental signals and to yield tissue grafts for further studies and eventual application in regenerative medicine. Our overall scientific goal is to obtain new critical information that will improve our understanding of the phenomena and mechanisms involved in human stem cell differentiation during cranio- and orofacial tissue development. The related practical goal is to establish in vitro systems that can be used to study the self-renewal and differentiation of stem cells in a manner predictable of their behavior in vivo, and to custom-design tissue grafts by directed differentiation of human stem cells.